Fish can be categorized by the water in which they spend their life cycle. Some spend their entire life cycle in salt water (generally marine fish). Herein, salt water is defined as water having a salt content about the same as that of sea water. This disclosure will refer to salt content by specific gravity (e.g., the salt content of sea water is about 1.022). Other fish spend their entire life cycle in fresh water, herein defined as having a salt content of about 1.000 specific gravity. Fresh-water fish can live in water having up to about 1.003 salt content. Some fish can live in brackish water. Although brackish water is generally regarded as water having a salt content of about 1.001 to about 1.003 specific gravity, this disclosure defines brackish water as water that has a salt content between about 1.000 and about 1.022. Fish that live in brackish water can usually also live in fresh or salt water. Still other fish spend part of their life cycle in fresh water and part of their life cycle in salt water (e.g., salmonid).
Heretofore, aquarists can keep salt-water fish only in salt-water aquariums. However, salt-water aquariums present many maintenance problems. Therefore, it is not surprising that the aquarium industry estimates that about 9 out of 10 aquarium hobbyists do not keep salt-water fish despite the fact that salt-water fish often have more decorative colors and shapes than other types of fish. A salt-water fish that can be maintained in either fresh water or brackish water would probably find commercial success.